Polyimide resins have been used for various purposes because of their heat resistance, flame retardance, chemical resistance, and high electrical insulation reliability. For example, they are high in insulation reliability and environmental stability in the form of films, and as such, they have been widely used as electrical insulating substrates and wire coating materials for use in various mobile phones. Further, they are excellent in abrasion resistance, heat resistance, and chemical resistance in the form of molded products and the like, and as such, they have been used as bearings and the like. Further, polyimide resins have been under research and development in the form of foam, fibers, and the like.
Among them, polyimide fibers are superior in high-temperature stability and chemical resistance to general organic polymeric resin fibers and therefore have been widely used as heat-resistant bag filters for use in exhaust gas treatment (e.g., see Patent Literatures 1 to 3), as a heat-resistant garment (e.g., see Patent Literature 4), and as base materials for various electrical insulating materials (e.g., see Patent Literature 5).
Widely used examples of polyimide fibers are filamentary polyimide fibers spun out of an organic-solvent soluble polyimide resin by a dry spinning method (e.g., see Patent Literatures 6 to 8).
Further known is a thermal insulating and sound-absorbing material for use in aircraft, including: non-thermoplastic fibers; and nodes composed of molten thermoplastic fibers, the nodes surrounding and thereby linking portions of adjoining non-thermoplastic fibers (e.g., see Patent Literatures 9 and 10).
Unfortunately, however, the assemblies of polyimide fibers as disclosed in Patent Literatures 1 to 8 are heavy in weight and insufficient in thermal insulation performance and, sound absorbency.
Specifically, the dry spinning method is a method including the steps of: discharging a spinning solution of a polymeric resin through a spinneret in a vertical direction; dry-removing a solvent from a surface by spinning in a high-temperature drying furnace; and spinning into filaments and winding the filaments. The resulting polyimide fibers are curveless in shape and therefore less likely to become tangled with each other. For this reason, an aggregate of polyimide fibers obtainable by the dry spinning method is high in bulk density, low in amount of air retained therein, and therefore low in thermal insulation performance. Furthermore, the high bulk density leads to an increase in weight of a product obtainable by using such a polyimide fiber aggregate.
Further, although the diameter of spun fibers can be made smaller by raising the draw ratio, such a decrease in diameter of the spun fibers makes the fibers likely to be broken in the middle, thus making the spun fibers lower in yield.
For example, even if polyimide fibers are formed into nonwoven fabric or felt as in the case of the formed article of polyimide fibers as disclosed in Patent Literature 1, the bulk density can only be approximately 0.5 to 1.0 g/cm3 (500 kg/m3 to 1,000 kg/m3).
Furthermore, the thermal insulating and sound-absorbing material for use in aircraft disclosed in Patent Literatures 9 and 10 is inferior in heat resistance.
Specifically, the thermal insulating and sound-absorbing material for use in aircraft disclosed in Patent Literatures 9 and 10 is hard to produce with use of filaments and therefore is formed with use of staples. That is, the thermal insulating and sound-absorbing material for use in aircraft realizes a low density through binding of short non-thermoplastic fibers via nodes composed of molten thermoplastic fibers. For this reason, the density becomes extremely high when the nodes are melted by heat and therefore the binding of the non-thermoplastic fibers is unbound.